I. Determine the maximum tolerated dose (MTD) of yttrium Y 90 ibritumomab tiuxetan (IDEC-90Y2B8) administered with rituximab with and without filgrastim (G-CSF) and interleukin-11 (IL-11) in patients with relapsed low-grade or follicular CD20+ non-Hodgkin's lymphoma. (Phase I) II. Determine the toxicity of this regimen in these patients. III. Determine the response rate in patients treated with this regimen. IV. Compare tumor and normal organ dosimetry with positron emission tomography and computerized tomography scans, subsequent tumor response, and normal organ toxicity by utilizing indium In 111 ibritumomab tiuxetan radioimmunoconjugate scans before each IDEC-90Y2B8 dose in these patients. (Phase I) V. Determine the immune response to this regimen, in terms of human anti-mouse and human anti-chimeric antibody formation, in these patients. (Phase I) VI. Determine whether G-CSF and IL-11 can ameliorate the effect of the MTD of IDEC-90Y2B8 on bone marrow function in these patients. (Phase I) VII. Determine progression-free survival at 3 years. (Phase II)